Isopycnal eddy mixing across the Kuroshio Extension: Stable versus unstable states in an eddying model

Abstract

The Kuroshio Extension (KE) jet transitions between stable and unstable states on interannual time scales. Cross‐jet eddy mixing in the two states is contrasted in the KE region ( 28°−40°N, 125°−165°E), using a global eddying 0.1° configuration of the Parallel Ocean Program with online numerical particles. The 4 year period chosen (June 1994 to May 1998) covers a full cycle of the stable state, unstable state and the transition period. Large values of cross‐jet eddy diffusivities within the KE jet are concentrated in the upper 1000 m. In the upper ocean, elevated cross‐jet mixing within the KE jet is mainly concentrated in the downstream part of the KE jet, where the jet is weak but eddy activity is strong. The simulated time‐mean KE jet is more intense and extends further east in the stable state than in the unstable state. Consequently, strong cross‐jet mixing within the KE jet is located west of 150°E during June 1996 to May 1997 (a typical unstable state), but east of 150°E during June 1995 to May 1996 (a typical stable state). However, average mixing within the KE jet is indistinguishable in the typical stable and unstable states. In the deep ocean, mixing is strongly influenced by topography, and thus their horizontal structures have less inter‐annual variability than in the upper ocean. One caveat is that results here cover one representative cycle of the two states. To obtain the climate mean mixing structures for the stable or unstable state, one would need numerical output covering a period much longer than 4 years.

Mixing Nonlocality and Mixing Anisotropy in an Idealized Western Boundary Current Jet

“…Oceanic jets are prevalent in the World Ocean and play an important role in the climate system. Jets in some oceanic regions are latent, that is, only manifest themselves in the time-mean flow field, such as those in the central Pacific (e.g., Maximenko et al 2005;Kamenkovich et al 2009;Berloff et al 2009;Chen et al 2015a). Others are intense and clearly visible in the flow snapshots, with notable examples in the energetic oceanic regions such as in the western boundary current (WBC) extensions and in the Antarctic Circumpolar Current (ACC).…”

“…Others are intense and clearly visible in the flow snapshots, with notable examples in the energetic oceanic regions such as in the western boundary current (WBC) extensions and in the Antarctic Circumpolar Current (ACC). These intense jets have extremely large along-jet transports, are an importance energy source to the eddy field through instability processes, and can serve as cross-jet transport barriers (Bower et al 1985;Gille et al 2007;Thompson 2008;Chen et al 2014aChen et al ,b, 2016. As a consequence, these jet flows greatly regulate the global tracer and energy budgets and thus climate variability.…”

“…Unsurprisingly, many studies exist about the dynamics and consequences of these strong jets. Topics include, but are not limited to, their spatiotemporal structures, interaction with topography, and energy exchange with eddies (e.g., Watts et al 1995;Killworth and Hughes 2002;Howe et al 2009;Jayne et al 2009;Thompson 2010;Qiu and Chen 2010;Venaille et al 2011;Waterman et al 2011;Thompson and Sallée 2012;Tracey et al 2012;Chen et al 2014a). In particular, transport and mixing across jets, as well as the role of jets in regulating cross-jet transport and mixing, have received much attention recently, as cross-jet mixing is critical to the meridional transport of heat and other tracers across the ACC and WBCs.…”

“…For example, Samelson (1992) evaluated the effect of the frequency of meander fluctuations in cross-jet exchange, and Rypina et al (2011) characterized the important role of the Gulf Stream in the fluid exchange between subtropical and subpolar gyres. Further, the horizontal and vertical structures of isopycnal eddy diffusivities across the jets have been estimated in the Southern Ocean, Kuroshio Extension (KE) regions, and idealized circumpolar current by multiple studies (e.g., Gille et al 2007;Abernathey et al 2010;Griesel et al 2014;Chen et al 2014b;LaCasce et al 2014;Wolfram and Ringler 2017). Much theoretical effort, based on the critical layer concept from Green (1970), has also been made toward quantifying the role of the jet in cross-jet mixing (e.g., Ferrari and Nikurashin 2010;Klocker et al 2012a;Bates et al 2014;Chen et al 2015b).…”

“…In particular, the factors controlling mixing anisotropy, anticipated to be large in zonally elongated, highly anisotropic systems like WBC jets, remain largely unclear. Furthermore, theories built upon the critical layer concept are mostly used to discuss the role of the jets in regulating cross-jet mixing, not the full diffusivity tensor (e.g., Green 1970;Ferrari and Nikurashin 2010;Klocker et al 2012a;Bates et al 2014;Chen et al 2015b). Motivated by these gaps, this study aims to estimate the full diffusivity tensor k ij and its corresponding mixing ellipses in an idealized model of a WBC jet and in particular to assess the role of the jet in shaping the anisotropy of the mixing.…”